The KDE panel supports so-called "tiled buttons". This means that the buttons shown on the panel will be drawn using configurable images. To enable button tiling, check the Enable background tiles option in the Buttons tab. Then you can configure tiles for certain kinds of buttons.
For every kind of panel button there's a frame offering an Enabled option to enable or disable tiled images. If tiles are enabled for this kind of button, you can choose a tile in the combo box below and the box on the right will show a preview of this tile.
The different kinds of buttons are:
The icon for the K Menu will be displayed as a tile
The icons for Quickbrowsers will be displayed as tiles
The icons which start applications will be displayed as tiles
The icons for legacy applications will be displayed as tiles
The Window List icon will be displayed as a tile
The Desktop Access icon will be displayed as a tile